This invention relates to an optical information reading apparatus such as an optical disk player, etc., and more particularly to an optical information reading apparatus in the form of an employment of a single light beam to conduct a tracking servo control utilizing interference of a reflected and diffracted light thereof.
In optical information reading apparatus such as a laser vision disk (LVD) player system, a compact disk (CD) player system, or a Direct Read After Write (DRAW) optical disk system, etc., a tracking control is used in order to precisely trace pits on the disk. As this tracking control system, various systems are proposed. As one of these systems, there is a method of detecting tracking information included in a transmitted light or a reflected light from the disk of a single light beam irradiated onto the disk in place of using a beam exclusive for tracking in addition to a beam for recording and playing back information.
The optical information reading apparatus using this tracking control system comprises a light emitting unit including a laser diode, etc., a light receiving unit including a quadrant-separated type photo detector, etc. and a signal processing unit. A laser beam emitted from the light emitting unit is reflected and diffracted by information recording pits on the optical disk, and is then incident to the light receiving unit. In the light receiving unit, respective subdetectors of the quadrant-separated type photo detector transform the incident light to electric signals. The signal processing unit makes a comparison between sum of electric signals from a pair of subdetectors in one diagonal direction and sum of electric signals from a pair of subdetectors in the other diagonal direction to detect the direction of a phase shift therebetween, i.e., discrimination between lead and lag of phase, and a quantity of shifts thereof. Thus, the signal processing unit makes a correction or modification in a direction opposite to the polarity (.+-.) of this shift and of a quantity equal to the quantity of shifts, thereby making it possible to conduct a tracking control.
However, in the above-described conventional optical reading apparatus, since there is employed the system of detecting a phase difference between sum signals of output signals in diagonal directions of the quadrant-separated photo detector, there was the problem that where there is any scratch or stain on the surface of the optical disk, deviation may take place on the track.
Namely, it is considered that when a laser beam is passed through a scratched or stained portion on the surface of the optical disk, light quantity is partially or entirely reduced by such a scratch or stain on the quadrant-separated photo detector.
For this reason, although there is no deviation between a laser beam and a track, a seeming tracking error signal is outputted, deviation takes place on the track.